The present invention relates to an ATM cell loss detecting and controlling apparatus in an ATM switching system for transferring an ATM cell.
An ATM (Asynchronous Transfer Mode) switching system is intended to transfer a cell having a 53-byte length including a 5-byte ATM header and a 48-byte data field and capable of performing a broad band transmission. According to this ATM switching system, it may happen that the cells are disposed of when the cells are lost or congregated. A desirable point in this system is that invalid holding or the like of a buffer memory is avoided by easily detecting such a cell loss.
The ATM switching system includes a plurality of ATM switching equipments interposed between a terminal device on a transmitting side and a terminal device on a receiving side, and pieces of data are stored in a packet known as a cell and thus transferred. The cell in that case is constructed of the 5-byte ATM header containing a virtual path identifier (VPI), a virtual channel identifier (VCI), a payload type (PT) and a cell loss priority indication (CLP) and the 48-byte data field containing pieces of data, etc.
Further, the ATM switching equipment performs a routing process on the basis of the virtual path identifier (VPI) and the virtual channel identifier (VCI). Also, when the cells are concentrated on the same route enough to be congregated, the cells are disposed of on the basis of headers, a quality-of-request-communication (QOS) and the cell loss priority indication (CLP) of the ATM header. Further, if an error is caused in the virtual path identifier (VPI) or the virtual channel identifier (VCI) of the ATM header due to a transfer error on an ATM line, there is brought about a loss of the ATM cell enough to make the data untransferable to a target terminal.
An ATM adaptation layer (AAL) can be roughly classified into a cell segmentation/administration/reassembly sublayer (SAR) in which the cell is segmented and reassembled and into a convergence sublayer (CS) in which frame error control and flow control are conducted. AAL types 1-5 are defined, and among these types, the AAL type 3 is of a connection type, while the AAL type 4 is of a connectionless type. A head cell or an intermediate cell or a last cell or a sole cell can be represented by a segment type (ST). Further, the AAL type 5 is of the connection type, and a function of the above segment type (ST) substitutes for the payload type (PT) of the ATM header.
Accordingly, on a receiving side, the segment type (ST) or the payload type (PT) is identified, and it is possible to discriminate whether the received cell is the last cell or not. Further, on the receiving side, the received ATM cell is segmented into the data field and the ATM header, and a frame is constructed by use of the data field. For this purpose, a buffer memory is provided, and the last cell is received, thus completing a receipt of a series of data. Hence, it follows that the reassembled frame in the buffer memory is transferred to a processing unit posterior thereto.
As explained above, in case the loss of the ATM cell is caused, and if this is a loss of an arbitrary number of intermediate cells among a plurality of intermediate cells which are indicated by the segment type (ST) or the payload type (PT), the loss thereof can be detected by final frame checking, and it follows that a retransmitting process based on a high-order application is to be executed.
This kind of retransmitting process is, however, very time-consuming, and, meanwhile, it is required that areas of the buffer memory be secured.
Further, if the last cell is lost, the receipt does not come to an end forever, and, hence, the areas of the buffer memory remain secured. This conduces to a problem in which an invalid holding state of the buffer memory happens often.
The present invention aims at providing a technology capable of detecting the loss of the ATM cell by a comparatively simple process.